1. Field of the Invention
The present invention relates to light modulators such as traveling-wave light modulators.
2. Related Art Statement
In the optical communication field, it is predicted that since communication capacity will drastically increase, the capacity of the light transmitting system needs to be enlarged. At present, the light transmission speed of 2.4 Gb/sec. has been put into practical use. However, as compared with the frequency band (about 200 THz) in which transmission can be effected through optical fibers, the practically employed level is merely one hundred thousandth at the maximum. What is important in drastically increasing the transmission capacity is to develop light modulation technology.
There is the possibility that a traveling-wave light modulator having lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), potassium lithium niobate (KLN), potassium titanyl phosphate (KTP) or gallium-arsenide (GaAs) used as an optical waveguide, which modulator has excellent characteristics, can realize a broad band width at a high efficiency. Lithium niobate and lithium tantalate are extremely excellent materials as a ferroelectric material, and favorably have large electro-optical coefficients and can control light within a short optical path.
Factors which suppress the modulation frequency of the traveling-wave light modulator include velocity mismatch, dispersion, and electrode power loss. Among them, since velocity mismatch and dispersion are principally determined by the structure of the traveling-wave light modulator, it is important to analyze the structure and make an appropriate design thereof. On the other hand, conductivity and surface skin effect of the material are important for the electrode power loss.
The concept of velocity mismatch is now further explained. In the traveling-wave light modulator, the velocity of the light propagating along the optical waveguide largely differs from that of an electric signal (microwave) propagating along the electrode. Assume that the velocity of light and that of the microwave propagating through the crystal are taken as Vo and Vm, respectively. For example, in the case of the LiNbO.sub.3 having planar type electrodes, the refractive index of the LiNbO.sub.3 single crystal is 2.15 (wavelength: 1.5 .mu.m), and the velocity of the light propagating through the optical waveguide is inversely proportional to the refractive index. On the other hand, the effective refractive index for a modulation wave is given by a square root of the dielectric constant near the conductor. The LiNbO.sub.3 single crystal is a uniaxial crystal, with a dielectric constant in the Z-axis direction of 28 and in the X-axis and Y-axis directions of 43. Therefore, even if an influence of air having the dielectric constant of 1 is taken into account, the effective refractive index of the LiNbO.sub.3 modulator having a conventional structure is about 4, which is about 1.9.times.2.14. Therefore, the velocity of the light wave is about 1.9 times as much as that of the modulation wave.
The upper limit of the bandwidth fm of the light modulation or the modulating velocity is proportional to the reciprocal of a difference in velocity between the light wave and the microwave. That is, fm=1/(Vo-Vm). Therefore, assuming that the power loss by electrode is zero, a limit is a bandwidth fm time the electrode length M=9.2 GHz.multidot.cm. Actually, it is reported that in a light modulator having an electrode length of M=2.5 mm, fm=40 GHz. The effect due to the limit of the operation speed becomes more conspicuous as the electrodes become longer. Therefore, a light modulator having a broad bandwidth and high efficiency is in demand.
Recently, it has been proposed in the case of an optical waveguide device, such as the optical waveguide-type high speed modulators and high speed switches, that the phase matching frequency between the light propagated through the optical waveguide and the modulating voltage applied from the outside is shifted to a higher side by tens of GHz through designing the configuration of an upper electrode on a substrate in a special shape or forming an accumulated layer of glass ("EO devices using LN" in "O plus E", May 1995, pp 91-97).
According to this literature, since the speed of the microwave is determined by the average value of the dielectric constant of an area through which electric forces pass between a thin signal electrode and an earth electrode, the modulating speed is increased by thickening the electrode and using a buffer layer composed of SiO.sub.2. Further, since the traveling-wave electrode constitutes a traveling passage, its characteristic impedance needs to be increased to around 50. In order to satisfy the above requirements, it is proposed that the electrode and the buffer layer be designed in a protruded shape, a hang-over shape, a grooved shape, a sealed shape or the like.
However, the thus constructed traveling-wave light modulator requires a complicated production process with a larger number of steps at a high cost. In addition, the optical waveguide must be kept aligned with the buffer layer, with the electrodes having the complicated configurations at a high accuracy. Furthermore, characteristics such as refractive index are likely to be changed by the formation of a working denatured layer due to damages in working, and according to a simulation result of an optical waveguide device, the characteristics are degraded and a light absorption characteristic and an extinction ratio characteristic become insufficient. Although the above difficult problems resulting from the production process are solved, it is still difficult to realize high speed modulation of not less than 10 GHz.multidot.cm.